Many patients with hypertrophic cardiomyopathy die suddenly and unexpectedly. Since many deaths may be due to arrhythmia, we used 24-hour ECG monitoring and treadmill exercise tests to determine the frequency and types of arrhythmias that occur in patients with this disease. More than 50 percent of patients in all subgroups (with or without either symptoms or left ventricular outflow tract obstruction) had multifocal and/or repetitive ventricular premature beats, including 19 percent who had ventricular tachycardia.